They can be dual-typed, but at least one of those has to match the type you choose. What type are you choosing, and what's your team look like?
I would have to choose Bug-type, of course. Team composition would be Galvantula, Yanmega, Heracross, Centiskorch, Genesect, and Rabsca. As to why... I just like 'em, lol. I'm not much of a competitive player. What would your team/type be?
I think I sticked to only using Fire type when I played one of the recent games, if only because a lot of my favorite pokemon (Ninetales, Arcanine, Houndoom, Salazzle) were that type.
Getting the TM for Flamethrower late in the game was a massive power spike. Type coverage isn't really an issue as well, because of sunny day + solarbeam.
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